The present invention relates to portable radio antennas, and more particularly to a process and a portable radio antenna for transmitting and receiving radio frequency signals in portable radio sets.
Generally, an antenna for use in a portable radio is constructed as a unitary structure, and is separately operated as a helical antenna or a rod antenna when the antenna is retracted or extended, respectively.
A conventional antenna, such as that disclosed in U.S. Pat. No. 4,868,576 entitled Extendable Antenna or Portable Cellular Telephones with Ground Radiator issued to Johnson, includes a helical coil and a half-wavelength radiator antenna. The radiator antenna is positioned on a top portion of a housing and includes two detents which engage tangs of an antenna housing when retracted and extended. The tangs snap into the detents, thereby providing the operator with tactile feedback indicating whether the antenna is fully retracted or extended. The radiator antenna slides into and out of the antenna housing, through the helical coil. The antenna also includes a protective top end cap, a top portion with the detent, a middle portion with a coil, a bottom portion with the detent and a bottom end cap. The radiator antenna is comprised of a flexible plastic material, such as "Delryn", with the mid-portion coil comprised of silver-plated beryllium-copper wire having a diameter of 13 mils (13/1000 inches).
When the radiator antenna is retracted, the unit is operated as a helical antenna. When extended, the extendable half-wavelength radiator antenna is capacitively coupled to the helical antenna. In order to capacitively connect the helical antenna to the radiator however, the length of the radiator must be extended. Consequently, the length of the radiator antenna becomes unnecessarily long. Also, since the center portion of the radiator antenna is comprised of conductive helical winding, the diameter of the radiator antenna is unaesthetically large.
One recent effort to overcome these disadvantages of Johnson '576 is found in the antenna described in Japanese Patent Provisional Publication No. 3-245603. This antenna includes a stainless wire rod antenna and a helical antenna positioned on a top portion of an antenna housing. A first feeder contains a housing connector positioned on a top portion of the housing; the first feeder is coupled to a stopper and to a metal ring connected to the housing connector. The metal ring is connected to a circuit board. When the antenna is extended, the stopper is connected to the housing connector. When the antenna is retracted, a second feeder coupled to the circuit board is connected to the stopper. This structure attempts to create an infinite antenna impedance.
Hence, when the antenna is extended, since the stopper is connected to the first feeder, the quarter-wavelength helical and rod antennas are operated as a single half-wavelength antenna. Since the radiating power distribution is large at the middle portion of the antenna however, the effective length and the gain of the antenna are reduced due to interference from the user's body. Also, since the helical antenna is positioned on a top portion of the antenna housing, it has little aesthetic appeal. Furthermore, such a set-up places the center of gravity of the radio set awkwardly high, so that the antenna swings and rotting noise begins, thus reducing the antenna's efficiency.
When the antenna is retracted, the stopper is connected to the second feeder, which in turn, is connected to the circuit board. Hence, the impedance of the rod antenna becomes infinite and therefore radiates no power. In this position, the helical antenna is connected to the first feeder and the helical antenna radiates power. Since a portion of the total radiating power is distributed to the retracted portion of the rod antenna however, actual radiating power is reduced.
Another notable effort in antenna development is disclosed in U.S. Pat. No. 5,204,687 entitled Electrical Device and Electrical Transmitter-Receiver particularly useful in a CT2 Cordless Telephone issued to Elliott et al. The device includes a quarter wavelength rod antenna carried by a housing, and a quarter wavelength antenna coil carried by one end of the antenna rod. The antenna rod is movably mounted through an opening in its housing to a retracted position wherein only the antenna coil is disposed externally of the housing, or to an extended position wherein the complete antenna rod and the antenna coil are disposed externally of the housing. In the retracted position however, only the antenna coil is operating, while in the extended position, only the antenna rod is operating. I have found therefore, the antenna is unable to continually maintain the characteristics of a rod antenna in both the retracted and extended positions.
U.S. Pat. No. 5,245,350 entitled Retractable Antenna Assembly with Retraction Inactivation issued to Sroka discloses another type of antenna assembly including an elongated radiating element which is movable between a retracted position and an extended position. The elongated element includes a central conductor which may be a solid rod antenna or, alternatively, may be in the form of a close-wound coil. It has been my observation however, that such an antenna assembly does not enjoy the versatility of being effectively operable as both a rod antenna and a helical antenna.
U.S. Pat. No. 5,258,772 entitled Antenna Device issued to Inanaga et al. mentions an antenna assembly including a retractable main antenna with a conducting coil spring connected to a base of the main antenna. When the main antenna is retracted into a housing, the coil spring is contracted and serves as a short antenna coil. It is my opinion however, that the gain of the antenna is reduced due to interference from the user's body because the coil spring is positioned within an interior portion of the housing.